refactor(tvix/eval): streamline construction of globals/builtins r/5581

Previously the construction of globals (a compiler-only concept) and
builtins (a (now) user-facing API) was intermingled between multiple
different modules, and kind of difficult to understand.

The complexity of this had grown in large part due to the
implementation of `builtins.import`, which required the notorious
"knot-tying" trick using Rc::new_cyclic (see cl/7097) for constructing
the set of globals.

As part of the new `Evaluation` API users should have the ability to
bring their own builtins, and control explicitly whether or not impure
builtins are available (regardless of whether they're compiled in or
not).

To streamline the construction and allow the new API features to work,
this commit restructures things by making these changes:

1. The `tvix_eval::builtins` module is now only responsible for
   exporting sets of builtins. It no longer has any knowledge of
   whether or not certain sets (e.g. only pure, or pure+impure) are
   enabled, and it has no control over which builtins are globally
   available (this is now handled in the compiler).

2. The compiler module is now responsible for both constructing the
   final attribute set of builtins from the set of builtins supplied
   by a user, as well as for populating its globals (that is
   identifiers which are available at the top-level scope).

3. The `Evaluation` API now carries a `builtins` field which is
   populated with the pure builtins by default, and can be extended by
   users.

4. The `import` feature has been moved into the compiler, as a
   special case. In general, builtins no longer have the ability to
   reference the "fix point" of the globals set.

This should not change any functionality, and in fact preserves minor
differences between Tvix/Nix that we already had (such as
`builtins.builtins` not existing).

Change-Id: Icdf5dd50eb81eb9260d89269d6e08b1e67811a2c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7738
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 105 insertions, 0 deletions

diff --git a/tvix/eval/src/compiler/import.rs b/tvix/eval/src/compiler/import.rs
new file mode 100644
index 000000000000..3a8847f2cbb4
--- /dev/null
+++ b/tvix/eval/src/compiler/import.rs
@@ -0,0 +1,105 @@
+//! This module implements the Nix language's `import` feature, which
+//! is exposed as a builtin in the Nix language.
+//!
+//! This is not a typical builtin, as it needs access to internal
+//! compiler and VM state (such as the [`crate::SourceCode`]
+//! instance, or observers).
+
+use std::rc::Weak;
+
+use crate::{
+    observer::NoOpObserver,
+    value::{Builtin, BuiltinArgument, Thunk},
+    vm::VM,
+    ErrorKind, SourceCode, Value,
+};
+
+use super::GlobalsMap;
+use crate::builtins::coerce_value_to_path;
+
+/// Constructs and inserts the `import` builtin. This builtin is special in that
+/// it needs to capture the [crate::SourceCode] structure to correctly track
+/// source code locations while invoking a compiler.
+// TODO: need to be able to pass through a CompilationObserver, too.
+// TODO: can the `SourceCode` come from the compiler?
+pub(super) fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builtin {
+    // This (very cheap, once-per-compiler-startup) clone exists
+    // solely in order to keep the borrow checker happy.  It
+    // resolves the tension between the requirements of
+    // Rc::new_cyclic() and Builtin::new()
+    let globals = globals.clone();
+
+    Builtin::new(
+        "import",
+        &[BuiltinArgument {
+            strict: true,
+            name: "path",
+        }],
+        None,
+        move |mut args: Vec<Value>, vm: &mut VM| {
+            let mut path = coerce_value_to_path(&args.pop().unwrap(), vm)?;
+            if path.is_dir() {
+                path.push("default.nix");
+            }
+
+            let current_span = vm.current_light_span();
+
+            if let Some(cached) = vm.import_cache.get(&path) {
+                return Ok(cached.clone());
+            }
+
+            let contents = vm.io().read_to_string(path.clone())?;
+
+            let parsed = rnix::ast::Root::parse(&contents);
+            let errors = parsed.errors();
+
+            let file = source.add_file(path.to_string_lossy().to_string(), contents);
+
+            if !errors.is_empty() {
+                return Err(ErrorKind::ImportParseError {
+                    path,
+                    file,
+                    errors: errors.to_vec(),
+                });
+            }
+
+            let result = crate::compiler::compile(
+                &parsed.tree().expr().unwrap(),
+                Some(path.clone()),
+                file,
+                // The VM must ensure that a strong reference to the
+                // globals outlives any self-references (which are
+                // weak) embedded within the globals.  If the
+                // expect() below panics, it means that did not
+                // happen.
+                globals
+                    .upgrade()
+                    .expect("globals dropped while still in use"),
+                &mut NoOpObserver::default(),
+            )
+            .map_err(|err| ErrorKind::ImportCompilerError {
+                path: path.clone(),
+                errors: vec![err],
+            })?;
+
+            if !result.errors.is_empty() {
+                return Err(ErrorKind::ImportCompilerError {
+                    path,
+                    errors: result.errors,
+                });
+            }
+
+            // Compilation succeeded, we can construct a thunk from whatever it spat
+            // out and return that.
+            let res = Value::Thunk(Thunk::new_suspended(result.lambda, current_span));
+
+            vm.import_cache.insert(path, res.clone());
+
+            for warning in result.warnings {
+                vm.push_warning(warning);
+            }
+
+            Ok(res)
+        },
+    )
+}